KENNEDY SPACE CENTER, FLA. - In the Vehicle Assembly Building (VAB), a segment of a Solid Rocket Booster (SRB) is lifted over the cross walk at the 16th level. This right aft center segment will be stacked with the aft booster on the other side that arrived in the VAB Nov. 22 for the Return to Flight mission STS-114. Two SRBs are stacked on the Mobile Launch Platform and later joined by the External Tank.. The twin 149-foot tall, 12-foot diameter SRBs provide the main propulsion system during launch. They operate parallel with the Space Shuttle main engines for the first two minutes of flight and jettison away from the orbiter with help from the Booster Separation Motors, about 26.3 nautical miles above the Earth?s surface.